1. Field of the Invention
This invention relates to a surface-acoustic-wave device functioning as a surface-acoustic-wave amplifier.
2. Description of the Prior Art
As disclosed in U.S. Pat. No. 4,233,530, the assignee of the present invention has developed a surface-acoustic-wave device as illustrated in FIG. 1, which is operative by a continuous wave and has a desired S/N ratio.
In FIG. 1, numeral 1 designates a semiconductor substrate, and an insulator film 2 and a piezoelectric layer 3 are laminated on the semiconductor substrate 1. A square pumping electrode 4 to which a DC bias voltage and a pumping voltage are applied and input and output transducers 5 and 6 are arranged on the piezoelectric layer 3.
Numeral 7 designates a DC source for applying a DC bias voltage, 8 is an inductor for AC blocking, 9 is a high-frequency power source for applying a pumping voltage, 10 is a capacitor for DC blocking, 11 is a matching circuit, 12 and 13 are surface-acoustic-wave absorbing members for preventing undesired reflection of surface acoustic wave at the ends of the device.
The DC bias voltage is applied from the DC power source 7 to the pumping electrode 4 so as to create a suitable depletion-layer capacitance at a surface portion of the semiconductor substrate 1 under the pumping electrode 4. Further, the pumping voltage having a frequency 2fo twice that of a center frequency fo of a desired frequency band is applied from the high-frequency power source 9 to the pumping electrode 4 so that the depletion layer capacitance is caused to oscillate and modulated at the frequency 2fo.
When an electric signal is applied to the broad-band input transducer 5, the input electric signal is converted into a surface-acoustic-wave signal which is propagated on the surface of the piezoelectric layer 3 rightwardly and leftwardly as viewed in FIG. 1.
In the course that a signal component of the surface-acoustic-wave input signal 15 propagating in the rightward direction, which has a frequency around fo, passes through an operating region under the pumping electrode 4, the piezoelectric potential thereof is subjected to a parametric interaction with the pumping voltage due to the depletion layer capacitance non-linearity effect on the surface of the semiconductor substrate 1 so that the component is amplified. This amplified surface-acoustic-wave signal 16 is converted into and outputted in the form of an electric signal by the output transducer 6.
At the same time, a surface-acoustic-wave signal 17, which has a frequency fi (fi=2fo-fs, fs: a frequency of the input signal) corresponding to the amplitude of the surface-acoustic-wave input signal 15, is also produced from the pumping electrode 4 and propagated leftwardly as viewed in FIG. 1. This surface-acoustic-wave signal 17 may also be outputted as an output signal.
In the surface-acoustic-wave device having the square pumping electrode 4 as described above, a circuit electrically equivalent to the condition beneath the pumping electrode 4 is as illustrated in FIG. 2. In FIG. 2, C.sub.1 is a capacitance of the piezoelectric layer 3, C.sub.2 is a capacitance of the insulating layer 2 and C.sub.3 is a depletion layer capacitance of a surface of the semiconductor substrate 1.
The relations between the aforesaid capacitances and the parametric interaction will now be discussed. The efficiency of the parametric interaction is enhanced as the capacitance C.sub.2 of the insulating film 2 is increased and when the depletion layer capacitance C.sub.3 is substantially equal to the capacitance C.sub.1 of the piezoelectric layer 3. The value of the capacitance C.sub.1 of the piezoelectric layer 3 per unit area is in inverse proportion to a thickness d of the piezoelectric layer 3, and the thickness d of the piezoelectric layer 3 is suitably determined in connection with a wavelength of a surface acoustic wave.
The value of the depletion layer capacitance C.sub.3 per unit area is determined by resistivity of a surface portion of the semiconductor substrate 1. In practical use, the value of the resistivity is restricted.
As described above, the capacitance C.sub.1 of the piezoelectric layer and the depletion layer capacitance C.sub.3 are determined by different factors, respectively. This makes it very difficult to maximize the efficiency of the parametric interaction at a desired frequency. In this respect, a further improvement has been awaited for the conventional surface-acoustic-wave device although it have excellent effects as described above.